There is a great interest in the nuclear energy industry for construction materials which will absorb, and therefore not release, neutrons, e.g. in containers for waste fuel. Boron carbide has been used for many years in the nuclear industry as a neutron absorbing material and is a commercially available material meeting ASTM standards. Boron carbide reinforced metal matrix composites also have application as lightweight structural materials.
International Application WO 00/26921 filed in the name of Reynolds Metals Company and published on May 11, 2000, describes the use of an aluminum alloy-boron carbide composite as a neutron absorbent material for storage of both hot section and spent nuclear fuel. These composite products were prepared by a powder metallurgical technique in which aluminum alloy powder was first mixed with boron carbide particles. The preferred aluminum alloy for the matrix was found to be an AA6000 series alloy, such as AA6010 alloy, which was mixed with at least 15% boron carbide particles. The AA6000 series alloys contain at least 0.25% Mg and
AA6010 contains at least 0.8% Mg. The reference found Al, Mg and Si to be acceptable elements, while finding AA2000, AA3000 and AA7000 alloys to be undesirable.
Powder metallurgy is an expensive technique for manufacturing large industrial components as required for the nuclear industry. There is, therefore, a need for a simpler and less expensive method for producing aluminum alloy-boron carbide composite products. Skibo et al. U.S. Pat. No. 4,786,467 describes a method of making aluminum alloy composites in which a variety of non-metallic particles are added to the aluminum alloy matrix. Among a wide variety of non-metallic particles that were mentioned, boron carbide was included. However, no tests are shown using boron carbide and the tests were conducted primarily with silicon carbide particles. In the Skibo method, the silicon carbide particles were mixed into a molten aluminum alloy and the mixture was then stirred to wet the aluminum alloy to the particles. The mixed material was then cast.
It was found that there can be problems of reaction between certain filler particles and the metal alloy matrix, such as is described in Hammond et al. U.S. Pat. No. 5,186,234. That patent was overcoming a problem encountered in certain situations where the molten composite material cast very poorly, had low fluidity and resulted in an unacceptable product. This was particularly a problem in a foundry remelter for holding molten composites containing SiC in an aluminum matrix.
It was found that certain alloying elements can inhibit wetting of the refractory particles in a metal matrix composite, such as is described in Skibo U.S. Pat. No. 5,083,602, in which case such wettability inhibiting elements were added after the alloy had wetted the particles. This did not address the problem of attack on the refractory by magnesium during wetting, since magnesium was described as useful in encouraging wetting in the first (wetting) step.
Lloyd et al. EP 0 608 299 describes a procedure where aluminum particles are dispersed in an aluminum alloy containing about 0.15 to 3% Mg where strontium is added to suppress the formation of spinal phase, which otherwise forms and depletes the matrix of available magnesium.
Hansson et al. U.S. Pat. No. 5,246,057 describes a procedure where alumina particles are dispersed in an aluminum alloy containing an initially high Mg concentration to produce a stable spinel coating on the alumina which is subsequently reduced to the desired magnesium level by dilution.
Ferrando et al. U.S. Pat. No. 5,858,460 describes a method of producing a cast composite for aerospace applications using boron carbide in a magnesium-lithium or aluminum-lithium alloy wherein a silver metallic coating is formed on the particle surfaces before mixing them into the molten alloy. This was done to overcome a problem of poor wettability of the particles by the alloy and reactivity.
Pyzik et al. U.S. Pat. No. 5,521,016 describes a method of. producing an aluminum-boron carbide composite by infiltrating a boron-carbide preform with a molten aluminum alloy. The boron carbide is initially passivated by a heat treatment process.
Rich et al. U.S. Pat. No. 3,356,618 describes a composite for nuclear control rods formed from boron carbide or zirconium diboride in various metals where the boron carbide is protected by a silicon carbide or titanium carbide coating applied, for example by chemical vapour deposition, before forming the composite. The matrix metals are high temperature metals however, and do not include aluminum alloys.
Jason S. H. Lo, CA 2,357,323 describes a composite for brake applications formed from a preform of refractory particles, whiskers or fibres which is infiltrated (e.g. by squeeze casting) with an aluminum alloy containing 1 to 40% binary intermetallic particles formed by adding a second metal powder to the aluminum alloy before infiltration. The intermetallic particles are formed both in the molten aluminum and also in heat treatments of the finished composite. The refractory particles include boron carbide and the second metal includes titanium.